The present invention relates to a caliper or sensing device for sensing the thickness of a signature being conveyed by a gripper mechanism and in particular to such a caliper for use in a signature handling apparatus for the assembly of signatures into various book forms. The invention is particularly adaptable to the operation where the signatures are opened into an inverted "V" shape for placement on a saddle gatherer prior to binding. The signatures, which in the simplest form are single sheets of paper folded in the middle so as to constitute four page surfaces and range to 16 sheets with 64 page surfaces, leave the folding operation with the page surfaces on the opposite sides of the center fold in contact. Before the pages can be bound, the halves, or laps, on the opposite sides of the fold must be spread so that the signatures can be assembled together and the folds of the signatures can be stapled or stitched.
The present invention was developed for use with a signature opener utilizing rotating drums but its use is not restricted to devices having rotating drums. A signature opener mechanism having rotating drums is described in U.S. Pat. No. 3,809,384. In brief, as the folded signature leaves a main drum it descends open end first between two drums, each of which clamp one half of the signature by means of vacuum and/or mechanical grippers. As the drums rotate in opposite directions, the signature laps are opened into an inverted "V" and released onto a triangular bar known as the saddle from which it is conveyed and gathered together with other signatures by a conveyor chain known as the saddle chain, and delivered to the saddle stitching machine for stapling. Machines of this general construction are known in the art as saddle gatherers or inserters.
Because improper spreading of the signature results in the loose, unbound pages, it is desirable to detect any misfeed as soon as practicable so as to avoid needless binding of defective books. Various calipering devices, such as mechanical cams, beta ray beam sensitometers and high voltage detectors, have been used in the prior art to gauge the thickness of an assembly of signatures after they have been collected on the saddle chain to detect a missing or improperly spread signature. All of these devices can be used with conventional magazines where thousands of magazines are published with the same thickness. However, in the future, innovative publishers who computerize the advertising content of their magazines so that the magazines can be custom built to the individual tastes of their readers will have magazines of varying thicknesses with various signatures contained therein. In a signature machine of the kind where individual signatures in folded form are transferred from hoppers and dropped onto a saddle where one signature is gathered on top of others to form a signature group, the proper delivery of a signature will have to be detected by single signature detection devices as the individual signatures are spread onto the saddle gatherer since it cannot be determined from the thickness of the signature group whether individual signatures of varying thicknesses have been properly spread thereon. The signature handling apparatus constructed in accordance with the present invention solves this problem as it detects the thickness of the individual signatures as they are transferred onto the saddle gatherer.
A single signature calipering device is also desirable with conventional signature gathering as it is more sensitive than a caliper which gauges a group of signatures assembled on the saddle gatherer, since as more pages are combined it is more likely variances in the thickness of paper will affect the reading. By detecting missing pages in single signatures before the signatures are grouped, this possibility of error is diminished.